In recent years, there has been much interest in multiple input, multiple output (MIMO) networks for enhancing data rates in wireless communication networks. MIMO networks employ multiple antennas at the transmitter and receiver to transmit and receive information. The receiver can exploit the spatial dimensions of the signal at the receiver to achieve higher spectral efficiency and higher data rates without increasing bandwidth.
To support MIMO transmissions, channel estimates are needed to characterize the effective channel for each spatial layer. In WCDMA networks, the base station transmits a pilot signal to the user terminals to enable channel estimation by the user terminals. The pilot signal is transmitted to all user terminals in a cell on a common pilot channel (CPICH) and serves two purposes. First, the pilot signals enable channel state information (CSI) estimation. Based on the CSI estimates, the user terminals generate a channel quality indication (CQI), and select a suggested rank and precoder for MIMO transmissions. Second, the user terminals use the pilot signal to generate channel estimates for coherent demodulation.
In WCDMA, common pilot signals are transmitted to all user terminals in a group or cell. Release 7 of the Wideband Code Division Multiple Access (WCDMA) standard defines two common pilot channels to support for 2×2 MIMO transmissions: the Primary Common Pilot Channel (P-CPICH) and Secondary Common Pilot Channel (S-CPICH). The S-CPICH is configured together with P-CPICH for channel estimation to enable 2×2 MIMO transmissions to MIMO-enabled user terminals.
In order to support 4×4 MIMO transmissions, it is necessary to obtain four channel estimates in order to characterize each of the spatial layers. Thus, to support four-branch MIMO transmissions, two additional common pilot channels need to be defined for channel estimation. The additional pilot channels need to be transmitted with sufficient power to ensure good channel estimates when a MIMO transmission of rank three or higher is scheduled for a compliant user terminal. However, the additional pilot channels will create additional interference.